1. Field of the Invention
The present invention relates to a process for preparing a carboxamide from a carbonyl compound and hydrogen cyanide. The present invention further relates to a process for preparing alkyl (meth)acrylates and polymers, and for producing moulding materials and polymer mouldings.
2. Description of the Background
The preparation of carboxamides by the hydrolysis of carbonitriles in the presence of a catalyst comprising manganese dioxide has been prior art for some time. Carboxamides are required as an intermediate in many cases in industry. For example, α-hydroxyisobutyramide can serve to prepare methacrylic acid or methacrylic esters, especially methyl methacrylate.
A particularly preferred process for preparing carboxamides is detailed in WO 2008/061822 A1.
Even though the teaching of the document detailed above already leads to a relatively inexpensive preparation, there is a permanent need to further improve the process. Typically, the cyanohydrin obtained by reaction of hydrogen cyanide and carbonyl compound, especially acetone, is stabilized by addition of acid. This stabilization has to be removed before the conversion of the cyanohydrin to the carboxamide, which is typically done by distillation. WO 2008/061822 A1 states that the mixture obtained after the reaction of acetone with hydrogen cyanide can be used for hydrolysis. However, it is not stated whether a purification has to be effected. In general, this purification comprises a two-stage distillation, the unconverted reactants being removed in a first stage. In a second stage, the acid used for stabilization is typically removed from the cyanohydrin. If no purification is performed, for example by distillation, relatively short catalyst service lives are obtained. In the case of use of a distilled reaction mixture, the service life of the catalyst can be improved considerably, although the overall efficiency of the process is worsened as a result owing to the energy consumption by the distillation.
In view of the prior art, it is thus an object of the present invention to provide processes for preparing carboxamides, which can be performed in a particularly simple and inexpensive manner and with a high yield. More particularly, a particular problem was to provide a process which, with a high rate, low energy use and low yield losses, ensures a particularly long lifetime of the catalyst and a long service life of the plant for preparing the carboxamide.
These objects and further objects which are not stated explicitly but are immediately derivable or discernible from the connections discussed herein by way of introduction are achieved by a process having all features of Claim 1. Appropriate modifications of the process according to the invention are protected in dependent claims. With regard to the process for preparing alkyl (meth)acrylates and polymers and for producing moulding materials and polymer mouldings, Claims 22, 24, 26 and 27 provide solutions to the problems underlying these objects.